1. Field of the Invention
The present invention relates to an image forming method, and more particularly to the improvement of image quality in an image forming method of a direct printing type, in which an image is formed by directly applying an aqueous ink to a recording medium by means of an inkjet recording apparatus.
2. Description of the Related Art
An inkjet recording apparatus is able to record images of good quality by means of a simple composition, and therefore such apparatuses are widely used as domestic printers for individual use and office printers for commercial use. In the case of office printers for commercial use, in particular, there are increasing demands for higher processing speed and higher image quality.
In improving the image quality achieved by an inkjet recording apparatus, generally, it is necessary that there should be little interference between ink droplets ejected from the nozzles of the ink head (hereinafter referred to as “landing interference”), little contraction of the image (hereinafter referred to as “image contraction”) and good reproducibility of text characters (hereinafter referred to as “text reproducibility”), and so on.
From the viewpoint of environmental suitability, water is widely used as the solvent in inkjet inks, and such aqueous inks using water as the ink solvent are liable to produce deformation of the recording medium, such as curling or cockling, due to permeation of the water into the recording medium during recording.
Currently, attention has focused on image forming methods based on a two-liquid system using an intermediate transfer body, as a way of improving image quality and suppressing curl in office printers. In an image forming method of this kind, ink and a treatment liquid that causes the ink to aggregate or precipitate are deposited onto an intermediate transfer body, thereby forming an ink aggregate body (desirably, the residual solvent is dried and driven off), whereupon the ink aggregate body is transferred to a recording medium. By forming an ink aggregate body on an intermediate transfer body, and then transferring same to a recording medium after drying the residual solvent in this way, images of high quality are obtained and curl becomes less liable to occur.
For example, Japanese Patent Application Publication No. 2004-010633 discloses an ink set used in a two-liquid method, which includes at least an aqueous ink containing a pigment, a water-soluble solvent and water, and a liquid composition that causes the aqueous ink to aggregate, wherein by making one of the aqueous ink and the liquid composition alkaline and the other acidic, it is possible to achieve excellent recording in terms of optical density, bleeding and color bleeding.
Japanese Patent Application Publication No. 11-188858 discloses an image forming method and apparatus which uses an intermediate transfer body, in which images which display little bleeding can be formed on a recording medium by depositing onto the intermediate transfer body a layer of powder (water-soluble resin) which is capable of swelling, increasing in viscosity and separating by reaction with the ink.
Japanese Patent Application Publication No. 2000-037942 discloses technology for improving optical density, bleeding, color mixing and drying duration, by controlling the aggregating properties of pigment on a recording medium through making one of a liquid composition (treatment liquid) and ink acidic and making the other alkaline.
However, since an image forming method which uses an intermediate transfer body is an indirect printing method which first forms an image (ink aggregate body) on an intermediate transfer body and then transfers this image to a recording medium, a greater number of steps are involved compared to a direct printing method which forms an image directly onto a recording medium, and hence the apparatus becomes correspondingly more complicated. Furthermore, improving image quality and suppressing curl in both indirect printing methods and direct printing methods is also important in terms of diversifying technology.
On the other hand, inkjet image recording methods are starting to spread from office use into various other fields. One of these is application in industrial printers. In the field of commercial printing, coated paper is mainly used as recording medium. The reason for using coated paper is the texture of the paper. Coated papers have slower liquid absorption and are therefore more liable to produce landing interference than normal papers or special inkjet papers, and furthermore, since they do not use a non-permeable medium as the base material, as in the case of laminated paper, such as special inkjet paper, then they are liable to produce curl if used in normal inkjet image formation.
In view of these circumstances, there is demand for an image forming method using a direct printing system of forming images by applying an aqueous ink directly onto a recording medium by means of an inkjet recording apparatus, which satisfies the conditions of producing little landing interference or image contraction, having good text reproducibility and making curl not liable to occur.